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# Precise magnetic guidance of metallic spheres2

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## Precise magnetic guidance of metallic spheres

(OP)
Hi Guys,

I am looking for a way to very precisely guide small metal spheres(or anything which can conduct current) into tissue for electro-chemical measurement purposes. For this to work I need a magnet able to stabilize a few micron small metal sphere over a working distance of at least 4 cm and with a positioning of at least 100 micron accuracy. In addition, within the two centimeters of tissue, I would like to place multiple metal spheres at different spots. The metal spheres should not move when the next sphere is put into place.

Is it possible to build something like this? Do you have any suggestions?

Thanks!

### RE: Precise magnetic guidance of metallic spheres

Does "a working distance of 4 cm" mean the sphere is at least 4 cm away from the magnet? While you still need 0.1 mm accuracy for this large distance? I feel it mostly impossible to accomplish.
Here are two examples:
1. assuming a Neo magnet with the dimension .06 dia x .03 thk. The magnetic field at 4 cm distance will be only 4.2Gs. When off center by 0.1mm (vertical angle =1 degree), the field will still be 4.2 Gs. You cannot locate the spheres
2. if using 1'' dia, 0.5 thk magnet. The magnetic field at 4 cm will be 3,782 Gs. if off center by .01 mm, the field will be a little stronger at 3,786 Gs. this field difference won't be able to guide your spheres either.

you could be able to make the ball permanent magnet, Micron sized ball offers nothing in magnetism at a distance of 4 cm.

### RE: Precise magnetic guidance of metallic spheres

This seems an impossible challenge. How would you even measure the position of the ball in the tissue with such precision? However, here is a concept:

First, magnetism can only attract a steel ball not repel it. Use magnetic pulses to "hammer" the pellet through tissue in tiny steps. Use feedback from position measurements to get to your final position.

### RE: Precise magnetic guidance of metallic spheres

The resolution does not sound practical.
And to hold location you would need an active system using sensors and electromagnets.
The real issue is that while you could use tapered pole pieces to create a localized strong field, the field is strongest at the pole pieces.
In order to keep something located between them is far different problem.
If you could work that part out then in order for them to not influence each other the separation would need to be a multiple of the 4cm working distance, and closer and 'cross-talk' would be inevitable.

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P.E. Metallurgy, Plymouth Tube

### RE: Precise magnetic guidance of metallic spheres

True. I forgot about the multiple pellets thing.

Also, if this is for electro-chemical measurements, don't you need a signal wire? Boron filaments are very stiff and can easily penetrate through tissue. The wire could be made steerable by putting a single bevel chisel point on the filament and rotating the filament.

### RE: Precise magnetic guidance of metallic spheres

(OP)
Thanks for your responses, I appreciate the help! I already imagined it was going to be quite a challenge.

I was picturing I would need a combination of multiple magnets, if so the space in between two magnets (with the sample in the middle) would have to be at least 4cm. That would at least 2cm away from the magnet.

The ball could be bigger than one micron, but preferably as small as possible. It could be something like 50-70 micron if that would help.

Also, indeed there would be a signal wire, but it shouldn't be magnetic (or at least orders of magnitude smaller then the sphere). This is because I want to have many spheres in the tissue and if the wiring is magnetic then everything would definitely move.

So I guess the most problematic part here is the distance and the 'focus'. In term of the 'focus', @ED how strong can we make the localized field? And what would be the minimal radius using these tapered poles? Is there any other way to redirect the magnetic field outside of the 100 micron?

As I mentioned, I would be able to scale up a bit, but this will hurt the spacial resolution of my measurements. So I would like to stay as small as possible. (in adition, a bigger ball will definitely damage the tissue more)

Regarding the wiring, Yes I am planning to use a ~1 micron wire and I was thinking about using a weak magnetic wire, since the wires and spheres will enter the tissue at the same location, so if the wire is as magnetic as the sphere then this will definitely create cross talk).

### RE: Precise magnetic guidance of metallic spheres

(OP)
In terms of steering/knowing the position: The tissue is quite homogeneous, so I would be able to calculate velocity per pulse for example, at different places using microscopy (this would be after the experiments). I was thinking of having a micro manipulator, which could keep the distance between the sphere and magnet consistent and therefore would always create the same velocity per pulse.

### RE: Precise magnetic guidance of metallic spheres

Remember magnets can only pull, they cannot push an object.
I suggest that you look into software that will let you model the magnetic fields.
You will see that getting the kind of field control that you are talking about will be very difficult.

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P.E. Metallurgy, Plymouth Tube

### RE: Precise magnetic guidance of metallic spheres

(OP)
I see! Ok, thanks for the information guys!

### RE: Precise magnetic guidance of metallic spheres

I think you need to divorce the motive force from the sensing means. Ultrasonics, near infrared, x-ray are all possible sensing means with the resolution you seem to want.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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